This invention relates to the high-temperature deposition of a powder onto a substrate and more particularly, to the control of the powder deposition to achieve a high-quality, dense deposit over an extended period of deposition.
The surfaces of articles are often subjected to extreme environmental conditions of temperature, corrosion, oxidation, wear, and the like. The base metal of the article is typically selected with mechanical properties such as strength, creep resistance, fatigue resistance, and the like in mind, and in many cases the base metal cannot withstand the surface environmental conditions. It is therefore common practice to protect the surfaces of the articles with a protective deposit or coating. The nature of the deposit is selected with consideration of the type of environmental conditions to which the article will be subjected in service.
In another application, an article may be made of a light-weight material that has adequate mechanical properties over most of its area, but inadequate mechanical properties in specific areas. Deposits may be applied in these areas to improve strength, fatigue resistance, creep resistance, and the like. In an example, a tungsten carbide/cobalt (WC/Co) hard-facing deposits are applied as stiffeners to titanium-alloy fan blades used in aircraft gas turbine engines.
There are many approaches to the deposition of relatively thin deposits on a substrate. The selection of an approach is made according to the nature of the material to be deposited, the nature of the substrate, the extent of the area to be coated, the required properties, the cost, and other considerations. In one popular deposition technology, a deposition apparatus generates a high temperature that at least partially melts the particles of a powder that is fed into the deposition apparatus. The mixture of hot gas and particles is projected out of the deposition apparatus and onto the surface of the article to be coated, where the melted portion solidifies to form an adherent coating.
When the coating must be of particularly high quality, the leading choice for such deposition is the detonation gun, or D-gun. In this device, a controlled explosion within the detonation gun produces a shock wave that partially melts the powder feed and propels it toward the substrate. The detonation gun has the disadvantage that it is large and heavy, and therefore must remain essentially fixed in location. The article to be coated must be moved to the proper position relative to the detonation gun. This requirement is troublesome when the article to be coated is large and itself difficult to manipulate. Additionally, it is desirable to improve upon the quality of the deposit over what may be accomplished with the detonation gun.
There is therefore a need for an improved high-temperature deposition approach. The present invention fulfills this need, and further provides related advantages.
The present invention provides a powder deposition apparatus and method that is highly controllable, is stable over extended periods, and uses a light-weight deposition gun that may be readily moved around an article being coated and is therefore amenable to robotic mounting and control. In studies leading to the present invention, it was determined that high-velocity oxyfuel (HVOF) powder deposition had the potential for a light-weight deposition gun and also the potential for producing high-quality deposits. The available HVOF deposition apparatus lacked sufficient controllability, leading to unacceptable quality of the deposits. The present invention provides for that controllability.
A powder deposition apparatus is operable to form a deposit on a deposition substrate. The powder deposition apparatus comprises a deposition gun having a combustion chamber wherein a mixture of a fuel and an oxidizer is burned to generate a pressurized deposition gas flow, a mixer wherein the pressurized deposition gas flow is mixed with a powder flow to form a deposition mixture flow, a deposition flow director that receives the deposition mixture flow from the mixer and directs the deposition mixture flow toward the deposition substrate, and a cooling structure operable with a flowing coolant (typically water) passing therethrough and in cooling communication with the mixer and with the deposition flow director. Using suitable sensors, an instrumentation array provides a fuel measurement of a flow rate of the fuel to the combustion chamber, an oxidizer measurement of a flow rate of the oxidizer to the combustion chamber, a powder measurement of a flow rate of a powder feed to the mixer, and a coolant measurement of a cooling capacity of the coolant. A deposition controller includes a controllable fuel source of the fuel communicating with the combustion chamber, wherein the controllable fuel source is automatically controlled responsive to the fuel measurement, and a controllable oxidizer source of the oxidizer communicating with the combustion chamber, wherein the controllable oxidizer source is automatically controlled responsive to the oxidizer measurement. A controllable powder source of the powder flow communicates with the mixer. The controllable powder source is automatically controlled responsive to the powder measurement. The deposition controller further includes a controllable coolant source of a flow of the coolant that provides an inlet flow of coolant to the cooling structure, wherein the controllable coolant source is automatically controlled responsive to the coolant measurement.
In one embodiment, the mixer comprises a central powder flow injector, and a set of deposition gas injectors arranged around a periphery of the central powder flow injector. The deposition flow director includes a barrel that receives the deposition mixture flow from the mixer, wherein the mixer is positioned at a first end of the barrel, and a powder spray nozzle positioned at a second end of the barrel opposite from the first end, wherein the powder spray nozzle is operable to project the deposition flow mixture toward the substrate. The cooling structure comprises a cooling jacket extending around at least a portion of the mixer and the deposition flow director.
Preferably, the controllable fuel source comprises a source of hydrogen gas, and the controllable oxidizer source comprises a source of oxygen gas. Most preferably, a flow ratio of the hydrogen gas to the oxygen gas is from about 2.2 to about 2.6. The controllable powder source comprises a source of a mixture of the powder entrained in a carrier gas. A most preferred powder is a mixture of tungsten carbide and cobalt powders.
In one version, the coolant measurement is a measured temperature of the flowing coolant, such as the measured temperature of the outlet flow of the coolant from the cooling structure. The deposition controller includes a heat exchanger that receives an outlet flow of the coolant, controllably cools the outlet flow of the coolant responsive to the measured temperature, and provides a cooled coolant flow to the cooling structure. The coolant measurement may instead be a measured flow rate of the coolant, and a flow controller provides the flow of the coolant responsive to the measured flow rate of the coolant.
Because of its small size and light weight, the deposition gun may be supported on and moved by a robotic head.
A method for forming a deposit on a deposition substrate comprises the steps of providing a deposition gun that burns a mixture of a fuel and an oxidizer to form a deposition gas flow, mixes a powder into the deposition gas flow to form a deposition mixture flow, and projects the deposition mixture flow therefrom. The deposition gun is provided with a flowing coolant. A flow rate of the fuel to the deposition gun, a flow rate of the oxidizer to the deposition gun, a flow rate of the powder to the deposition gun, and a cooling capacity of the coolant flow are all measured. The method includes set-point controlling the flow rate of the fuel, the flow rate of the oxidizer, the flow rate of the powder, and the cooling capacity of the coolant flow, all responsive to the step of measuring. Other compatible features of the invention as described herein may be used in conjunction with the method.
The present approach provides a deposition technology whose deposits are comparable in quality with, and sometimes superior to those of, detonation-gun technology. The present approach uses a light-weight deposition gun that is far more movable than the detonation gun, and accordingly allows the deposition gun to be moved rather than the article. Existing deposition technology was found to have the drawback, however, that it was closely dependent upon operating parameters such as fuel, oxidizer, and powder flow, and the cooling capacity of the coolant. The feedback control technique of the present invention increases the time stability of the deposition technique by controlling these parameters to set-point values.